Infrared kinetic spectroscopy of small free radicals

Abstract

The technique of infrared kinetic spectroscopy has been used to study the production of the propargyl radical from the reaction of singlet methylene with acetylene. The rate constant for this product channel was measured as \rm(3.5\pm0.7)\times10\sp{-10}\ cm\sp3\ molecule\sp{-1}\ s\sp{-1} at 295 K, relative to the known rate for \rm\sp1CH\sb2 with H\sb2 or CH\sb4. Methylene was produced in the singlet state by excimer laser photolysis of ketene at 308 nm in the presence of acetylene and either \rm H\sb2 or \rm CH\sb4. Reaction of \sp1CH\sb2 with acetylene produces propargyl, and reaction of \sp1CH\sb2 with either \rm H\sb2 or CH\sb4 produces CH\sb3. The intensity of a propargyl infrared absorption line was compared with that of a methyl infrared absorption line, and the rate of formation of propargyl was determined from the ratio of these two intensities and the known rates of reaction of singlet methylene. The relative peak infrared absorption cross-sections of methyl and propargyl were calibrated under the conditions of the experiment by photolyzing crotyl bromide at 193 nm to produce methyl and propargyl in equal concentrations. The reaction of the cyanomethylene (HCCN) radical with nitric oxide (NO) and oxygen (O\sb2) has also been investigated using infrared kinetic spectroscopy. The overall rate constants for each reaction have been determined. For the reaction with NO, the rate constant was determined to be \rm(3.5\pm0.6)\times10\sp{-11}\ cm\sp3\ molecule\sp{-1}\ s\sp{-1}, for reaction with O\sb2, the rate constant was \rm(1.8\pm0.4)\times10\sp{-12}\ cm\sp3\ molecule\sp{-1}\ s\sp{-1}. Several products were observed, but not quantified for either reaction. In the case of reaction with NO, hydrogen cyanide (HCN) and fulminic acid (HCNO) were observed. During reaction with O\sb2, HCN, hydrogen isocyanide (HNC) and carbon dioxide (CO\sb2) were observed. Species looked for but not observed include isocyanic acid (HNCO), ethynyl (C\sb2H) radical, cyanic acid (HOCN) and isofulminic acid (HONC). Under the conditions of our experiments, no reaction was observed between HCCN and the following: methane (CH\sb4), CO\sb2, acetylene \rm(C\sb2H\sb2), ethylene \rm(C\sb2H\sb4), carbon monoxide (CO) and hydrogen (H$\sb2).